Video decoding apparatus, video decoding method, and digital audio/video playback system capable of controlling presentation of sub-pictures

ABSTRACT

A video decoding apparatus capable of controlling presentation of sub-pictures includes a first decoder, a second decoder, a first scaler, a second scaler and a combiner. The first decoder and the second decoder respectively decode a digital audio/video signal to generate a decoded video and a decoded sub-picture. In accordance with an output picture size, the decoded video size, the decoded sub-picture size and a sub-picture aspect ratio, the first scaler and the second scaler generate a first scaling factor and a second scaling factor. The combiner combines the decoded video and the decoded sub-picture according to the first scaling factor and the second scaling factor and outputs the combination thereof. A video decoding method and a digital audio/video playback system capable of controlling presentation of sub-pictures are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation reissue of application Ser. No.13/689,769, filed on Nov. 30, 2012, which is an application for reissueof U.S. Pat. No. 7,844,168, filed on Jun. 20, 2006 and issued on Nov.30, 2010, which claims the priority of the prior-filed Taiwan patentapplication No. 94122087, filed on Jun. 30, 2005. The entirety of eachof the above-mentioned applications and patents is hereby incorporatedby reference and made a part of this specification.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention relates to a video decoding apparatus, a video decodingmethod and a digital audio/video playback system and, more particularly,to a video decoding apparatus, a video decoding method and a digitalaudio/video playback system capable of controlling presentation ofsub-pictures.

b) Description of the Related Art

Due to the advance of information technology, both audio and video datacan be stored, transmitted and played back in digital format. Besidesproviding movies of better quality, audio and video data in digitalformat offers many additional functions. Take the digital versatile disc(DVD) for instance, not only can the DVD store high-quality movies, butthe DVD can also store multilingual audio tracks and multiplesub-pictures for users to choose from during playback. One of the majorfunctions of sub-pictures is to act as subtitles, for example, anEnglish film can include therein English and Chinese audio tracks, andEnglish and Chinese subtitles, and during playback, users can choose theEnglish audio track with Chinese subtitle, or the English audio trackwith English subtitle, as well as the Chinese audio track with Englishor Chinese subtitles. Therefore, this function facilitatesinternationalization of films.

As common practice, even though the DVD contains multiple subtitles, theDVD is usually played back with only one subtitle displayed. If two ormore subtitles are displayed simultaneously, there can be overlay amongthe subtitles, making them difficult to read, or only one of thesubtitles is displayed in the overlaid region such that other subtitlescannot be completely displayed.

Furthermore, differently formatted DVD films can cause incompletedisplay of the subtitle during playback. For example, in Phase AlternateLine (PAL) standard adopted by most European countries and Australia,the resolution of the DVD film is 720×576 pixels. In National TelevisionSystem Committee (NTSC) standard adopted by North America and Japan, theresolution of the DVD film is 720×480 pixels. Therefore, when a DVD filmrecorded according to PAL standard is played back in a playback systemcompliant with NTSC standard, there is a possibility that the subtitleat the lower end of the picture is cut off and cannot be displayedcompletely. In addition, similar problems exist when playback systemswith different formats, namely 4:3 or 16:9, are used to play differentlyformatted DVD films.

In summary, it would be desirable if the sub-pictures can be completelydisplayed and overlay among multiple sub-pictures can be avoided duringplayback of a digital film.

SUMMARY OF THE INVENTION

In view of the problems mentioned above, an object of the invention isto provide a video decoding apparatus, a video decoding method and adigital audio/video system capable of controlling presentation ofsub-pictures, and the apparatus, method and system are able to controlthe aspect ratio and the position of the sub-pictures so that thesub-pictures can be completely displayed and any overlay among multiplesub-pictures is avoided.

To achieve the object mentioned above, the digital audio/video playbacksystem capable of controlling presentation of sub-pictures of theinvention decodes a digital audio/video signal, and to playback anoutput picture and a decoded audio through a video output device and anaudio output device. The digital audio/video playback system includes asignal loader, a signal parser, a video decoding apparatus and an audiodecoder. The signal loader is utilized to receive the digitalaudio/video signal. The signal parser is utilized to divide the digitalaudio/video signal into a video bit stream, a sub-picture bit stream andan audio bit stream. The audio decoder receives the audio bit stream,decodes it to a decoded audio, and outputs the decoded audio to theaudio output device. The video decoding apparatus receives the video bitstream and the sub-picture bit stream, decodes and combines them into apicture, and outputs the combined picture to the video output device toplayback.

The abovementioned video decoding apparatus includes: a first decoderwhich receives the video bit stream and decodes it in order to generatea decoded video; a second decoder which receives the sub-picture bitstream and decodes it in order to generate at least one decodedsub-picture; a first scaler which captures the size of the decoded videoand generates a first scaling factor according to the size of the outputpicture and the size of the decoded video; a second scaler whichcaptures the size and the position of the decoded sub-picture andgenerates a second scaling factor according to the size of the outputpicture, the size of the decoded sub-picture and a sub-picture aspectratio; and a combiner which sets the size of the decoded video accordingto the first scaling factor, sets the size and the position of thedecoded sub-picture according to the second scaling factor, combines thedecoded video and the decoded sub-picture, and outputs the combinedpicture to the video output device.

The invention also discloses a video decoding method capable ofcontrolling presentation of sub-pictures, which is utilized to decode adigital audio/video signal and to playback an output picture through avideo output device. The video decoding method includes: receiving avideo bit stream and decoding it to generate a decoded video; receivinga sub-picture bit stream and decoding it to generate at least onedecoded sub-picture; capturing the size of the decoded video andgenerating a first scaling factor according to the size of the outputpicture and the size of the decoded video; capturing the size and theposition of the decoded sub-picture, and generating a second scalingfactor according to the size of the output picture, the size of thedecoded sub-picture and a sub-picture aspect ratio; and setting the sizeof the decoded video according to the first scaling factor, setting thesize and the position of the decoded sub-picture according to the secondscaling factor, combining the decoded video and the decoded sub-picture,and outputting the combined picture to the video output device.

The video decoding apparatus, method and the digital audio/videoplayback system capable of controlling presentation of sub-picturesaccording to the invention are able to control the aspect ratio and theposition of sub-pictures in order to completely display sub-pictures. Atthe same time, when a single sub-picture or a plurality of sub-picturesare displayed, there is no overlay on the main video region and overlayamong the sub-pictures is avoided effectively so that each of thesub-pictures is completely displayed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a block diagram of a digital audio/video playback systemcapable of controlling presentation of sub-pictures according to apreferred embodiment of the invention.

FIG. 2 shows a video decoding method capable of controlling presentationof sub-pictures according to a preferred embodiment of the invention.

FIG. 3 shows a flow chart of a first scaler generating a first scalingfactor.

FIG. 4 shows a flow chart of a second scaler generating a second scalingfactor; and

FIG. 5 shows a flow chart of a combiner combining the decoded video andthe decoded sub-picture.

DETAIL DESCRIPTION OF THE INVENTION

The video decoding apparatus, video decoding method and digitalaudio/video playback system capable of controlling presentation ofsub-pictures according to preferred embodiments of the invention will beexplained below with reference to the drawings, wherein the likenumerals denote the like components.

Referring to FIG. 1, a digital audio/video playback system 1 capable ofcontrolling presentation of sub-pictures according to a preferredembodiment of the invention is utilized to decode a digital audio/videosignal (DAVS) and to playback an output picture and a decoded audiothrough a video output device 21 and an audio output device 22, whereinthe DAVS includes a video bit stream (VBS), a sub-picture bit stream(SPBS) and an audio bit stream (ABS). The digital audio/video playbacksystem 1 includes a signal loader 11, a signal parser 12, a videodecoding apparatus 13 and an audio decoder 14.

The signal loader 11 is utilized to receive DAVS. Take a DVD player forinstance, the signal loader 11 may include elements like a control unit,a loading unit, a signal processing unit and more. As for Digital VideoBroadcasting (DVB), the signal loader 11 may include elements like acontrol unit, an antenna, a signal processing unit and more. Therefore,in correspondence to different requirements, the signal loader 11 can berealized through different designs by a person skilled in the art, andthus the signal loader 11 is not explained in further detail.

The signal parser 12 is utilized to divide the loaded DAVS into a VBS, aSPBS and an ABS. The audio decoder 14 is utilized to receive the ABS andto decode it into a decoded audio (DA) for the audio output device toplayback.

The video decoding apparatus 13 is utilized to receive and decode theVBS and the SPBS, and then the video decoding apparatus 13 combines thetwo into a picture for the video output device 21 to playback. The videodecoding apparatus 13 includes: a first decoder 131, a second decoder133, a first scaler 132, a second scaler 134 and a combiner 135. Thefirst decoder 131 receives the VBS, decodes the VBS and outputs adecoded video (DV). The second decoder 133 receives the SPBS, decodesthe SPBS and outputs at least one decoded sub-picture (DSP) under thecontrol of the user's choice; for example, the second decoder 133 canoutput one or a plurality of DSPs. The first decoder can be an MPEGdecoder, while the second decoder can be a run-length decoder.

The first scaler 132 is utilized to capture the size of the DV, such asthe horizontal width and the vertical height, and to generate a firstscaling factor according to the size of the output picture and the sizeof the DV. The second scaler 134 is utilized to capture the size and theposition of the DSP, such as the horizontal coordinate and the verticalcoordinate, and to generate a second scaling factor according to thesize of the output picture, the size of the DSP and a sub-picture aspectratio. The combiner 135 sets the size of the DV according to the firstscaling factor, sets the size and the position of the DSP according tothe second scaling factor, combines the DV and the DSP, and outputs thecombined picture to the video output device 21 to playback. Theprocedures for generating the first scaling factor and the secondscaling factor, and for combining the DV and the DSP will be describedlater.

Referring to FIG. 2, a video decoding method capable of controllingpresentation of sub-pictures according to a preferred embodiment of theinvention is described. First, a VBS is received and decoded to generatea DV (S31), and then a SPBS is received and decoded to generate at leastone DSP (S32). Next, the size of the DV is captured, and a first scalingfactor is generated according to the size of an output picture and thesize of the DV (S33).

Referring to FIG. 3, the procedure for generating the first scalingfactor is described. First, the size of the DV, namely its horizontalwidth Vh and its vertical height Vv, is obtained (S331), and the size ofthe active display area of a video output device 21, namely itshorizontal width Dh and its vertical height Dv, is obtained (S332). Thenext step is to determine whether or not a film to be played supportsspecific video aspect ratios (S333), for example the aspect ratios ofmodes such as letterbox or pan & scan. If the film to be played does notsupport a specific display mode, the size of the output picture is equalto the size of the active display area of the video output device 21,namely the horizontal width OVh of the output picture is equal to thehorizontal width Dh of the active display area of the video outputdevice 21, and the vertical height OVv of the output picture is equal tothe vertical height Dv of the active display of the video output device21 (S335). If the film to be played supports a specific display mode,and the user chooses the specific display mode for playback, thehorizontal width OVh of the output picture is the product of thehorizontal width Dh of the active display area of the video outputdevice 21 and a horizontal aspect ratio VARh of the video aspect ratio,and the vertical height OVv of the output picture is the product of thevertical height Dv of the active display area of the video output device21 and a vertical aspect ratio VARv of the video aspect ratio (S334).Finally, the first scaling factor is calculated according to the size ofthe output picture and the size of the DV, and the first scaling factorincludes a first horizontal scaling factor (SF1h) and a first verticalscaling factor (SF1v). The SF1h is equal to the horizontal width OVh ofthe output picture divided by the horizontal width Vh of the DV. TheSF1v is equal to the vertical height OVv of the output picture dividedby the vertical height Vv of the DV.

Referring to FIG. 2 again, the step S34 is to capture the size and theposition of the DSP, and to generate a second scaling factor accordingto the size of the output picture, the size of the DSP and a sub-pictureaspect ratio.

Referring to FIG. 4, the procedure for generating the second scalingfactor is described. First, the size and the position of the DSP, namelyits horizontal width SPh, its vertical height SPv, its horizontalcoordinate, and its vertical coordinate, are obtained (S341). Then, thesub-picture aspect ratio of the DSP, which includes a horizontal aspectratio (SPARh) and a vertical aspect ratio (SPARv), is obtained (S342).The next step is to determine whether the film to be played permitssub-picture scaling (S343); scaling the sub-picture can result in ablurred sub-picture during playback, thus restriction on sub-picturescaling may have been added when the film was recorded. If sub-picturescaling is prohibited, a second horizontal scaling factor (SF2h) and asecond vertical scaling factor (SF2v) of the second scaling factor areboth 1 (S345), namely the sub-picture is not scaled. If the sub-picturescaling is allowed for playback, the SF2h is equal to the SF1h dividedby the SPARh of the sub-picture aspect ratio, and the SF2v is equal tothe SF1v divided by the SPARv of the sub-picture aspect ratio.

Referring to FIG. 2 again, finally, the size of the DV is set accordingto the first scaling factor, the size and the position of the DSP is setaccording to the second scaling factor, the DV and the DSP are combined,and the combined picture is output to the video output device 21 toplayback (S35).

The flow of combining the DV and the DSP is depicted in FIG. 5. Firstly,determine whether the DSP is for the purpose of visual effect instead ofbeing as the sub-title (S351). If yes, the relative positions betweenthe DSP and the DV should be fixed to retain the intended visual effect,thus, the size of the DSP is set so as to precisely achieve the purposeof visual effect. If the DSP is for the purpose other than visualeffect, for example as the sub-title, the size and the position of theDSP are set (S353). Next, whether the DSP exceeds the border of thepicture is determined (S354); if yes, the size and the position of theDSP are re-set (S353). Then, whether the DSP covers the main region ofthe DV is determined (S355). For example, generally the central area ofa DV is the main region, thus efforts should be made to prevent DSP fromcovering the central area of the DV as much as possible. If the mainregion of the DV is covered, the size and the position of the DSP arere-set as well (S353). If the user chooses to display a plurality ofDSPs, it is necessary to further determine whether there is overlayamong the plurality of DSPs (S356) when combining the DV and the DSPs.And if there is overlay among the plurality of DSPs, go back to stepS353 to re-set the sizes and the positions of the plurality of DSPs.After the sizes and the positions of all the DSPs are set (S357),combine the DV and the DSP, and output the combined picture to the videooutput device 21 for playback (S358).

Regardless of what the modes the film to be displayed supports and whatstandards the display system complies with, the video decodingapparatus, video decoding method and digital audio/video system capableof controlling the presentation of the sub-pictures according to theinvention are able to control the aspect ratio and the position of thesub-pictures so as to completely display the sub-pictures. Additionally,when a single sub-picture is displayed, the main video region is notcovered, and when a plurality of sub-pictures are displayed, overlayamong the plurality of sub-pictures is avoided to completely display allthe sub-pictures.

While the invention has been described by way of example and in terms ofthe preferred embodiment, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangementsapparent to those skilled in the art. For example, in the DVB system,the size and the coordinates of each object can be captured, and thesize and the position of each object can be properly set for it tofunction properly. Therefore, the scope of the appended claims should beaccorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

What is claimed is:
 1. A video decoding apparatus capable of controllingpresentation of sub-pictures, which is for decoding a digitalaudio/video signal and outputting an output picture through a videooutput device, wherein the digital audio/video signal includes a videobit stream and a sub-picture bit stream, the video decoding apparatuscomprising: a first decoder which receives the video bit stream anddecodes the video bit stream to generate a decoded video; a seconddecoder which receives the sub-picture bit stream and decodes thesub-picture bit stream to generate at least one decoded sub-picture; afirst scaler which captures the size of the decoded video and generatesa first scaling factor according to the size of the output picture andthe size of the decoded video; a second scaler which captures the sizeand the position of the decoded sub-picture and generates a secondscaling factor according to the size of the output picture, the size ofthe decoded sub-picture, and a sub-picture aspect ratio; and a combinerwhich sets the size of the decoded video according to the first scalingfactor, sets the size and the position of the decoded sub-pictureaccording to the second scaling factor, combines the decoded video andthe decoded sub-picture, and outputs the combined picture to the videooutput device.
 2. The video decoding apparatus as specified in claim 1,wherein: the first scaling factor comprises a first horizontal scalingfactor and a first vertical scaling factor, in which the firsthorizontal scaling factor is equal to the horizontal width of the outputpicture divided by the horizontal width of the decoded video, and thefirst vertical scaling factor is equal to the vertical height of theoutput picture divided by the vertical height of the decoded video; andthe second scaling factor comprises a second horizontal scaling factorand a second vertical scaling factor while the sub-picture aspect ratioincludes a horizontal aspect ratio and a vertical aspect ratio, in whichthe second horizontal scaling factor is equal to the first horizontalscaling factor divided by the horizontal aspect ratio of the sub-pictureaspect ratio, and the second scaling factor is equal to the firstvertical scaling factor divided by the vertical aspect ratio of thesub-picture aspect ratio.
 3. The video decoding apparatus as specifiedin claim 2, wherein the horizontal width of the output picture is theproduct of the horizontal width of the active display area of the videooutput device and a horizontal aspect ratio of a video aspect ratio, andthe vertical height of the output picture is the product of the verticalheight of the active display of the video output device and a verticalaspect ratio of the video aspect ratio.
 4. The video decoding apparatusas specified in claim 1, wherein if the decoded sub-picture is for thepurpose of visual effect, the combiner only sets the size of the decodedvideo and the size of the decoded sub-picture when combining the decodedvideo and the decoded sub-picture.
 5. The video decoding apparatus asspecified in claim 1, wherein when the decoding of the sub-picture bitstream generates a plurality of decoded sub-pictures, the combinerfurther sets the positions of the plurality of decoded sub-pictures soas to avoid overlay among the plurality of decoded sub-pictures.
 6. Thevideo decoding apparatus as specified in claim 1, wherein the firstdecoder is an MPEG decoder and the second decoder is a run-lengthdecoder.
 7. A digital audio/video playback system capable of controllingpresentation of sub-pictures, which is for decoding a digitalaudio/video signal, outputting an output picture through a video outputdevice, and outputting a decoded audio through an audio output device,the digital audio/video playback system comprising: a signal loaderwhich receives the digital audio/video signal; a signal parser whichdivides the digital audio/video signal into a video bit stream, asub-picture bit stream, and an audio bit stream; an audio decoder whichreceives the audio bit stream, decodes the audio bit stream to a decodedaudio, and outputs the decoded audio to the audio output device; and avideo decoding apparatus, including: a first decoder which receives thevideo bit stream and decodes the video bit stream to generate a decodedvideo; a second decoder which receives the sub-picture bit stream anddecodes the sub-picture bit stream to generate at least one decodedsub-picture; a first scaler which captures the size of the decoded videoand generates a first scaling factor according to the size of the outputpicture and the size of the decoded video; a second scaler whichcaptures the size and the position of the decoded sub-picture andgenerates a second scaling factor according to the size of the outputpicture, the size of the decoded sub-picture, and a sub-picture aspectratio; and a combiner which sets the size of the decoded video accordingto the first scaling factor, sets the size and the position of thesub-picture according the second scaling factor, combines the decodedvideo and the decoded sub-picture, and outputs the combined picture tothe video output device.
 8. The digital audio/video playback system asspecified in claim 7, wherein: the first scaling factor comprises afirst horizontal scaling factor and a first vertical scaling factor, inwhich the first horizontal scaling factor is equal to the horizontalwidth of the output picture divided by the horizontal width of thedecoded video, the first vertical scaling factor is equal to thevertical height of the output picture divided by the vertical height ofthe decoded video; and the second scaling factor comprises a secondhorizontal scaling factor and a second vertical scaling factor while thesub-picture aspect ratio includes a horizontal aspect ratio and avertical aspect ratio, in which the second horizontal scaling factor isequal to the first horizontal scaling factor divided by the verticalaspect ratio of the sub-picture aspect ratio, and the second verticalscaling factor is equal to the first vertical scaling factor divided bythe vertical aspect ratio of the sub-picture aspect ratio.
 9. Thedigital audio/video playback system as specified in claim 8, wherein thehorizontal width of the output picture is the product of the horizontalwidth of the active display area of the video output device and ahorizontal aspect ratio of a video aspect ratio, and the vertical heightof the output picture is the product of the height of the active displayarea of the video output device and a vertical aspect ratio of the videoaspect ratio.
 10. The digital audio/video playback system as specifiedin claim 7, wherein if the decoded sub-picture is for the purpose ofvisual effect, the combiner only sets the size of the decoded video andthe size of the decoded sub-picture when combining the decoded video andthe decoded sub-picture.
 11. The digital audio/video playback system asspecified in claim 7, wherein when the decoding of the sub-picture bitstream generates a plurality of decoded sub-pictures, the combinerfurther sets the positions of the plurality of decoded sub-pictures soas to avoid overlay among the plurality of decoded sub-pictures.
 12. Thedigital audio/video playback system as specified in claim 7, wherein thefirst decoder is an MPEG decoder, and the second decoder is a run-lengthdecoder.
 13. The digital audio/video playback system as specified inclaim 7, wherein the digital audio/video playback system is a DVDplayback system or a digital video broadcasting playback system.
 14. Avideo decoding method capable of controlling presentation ofsub-pictures, which is for decoding a digital audio/video signal,outputting an output picture through a video output device, wherein thedigital audio/video signal includes a video bit stream and a sub-picturebit stream, the video decoding method comprising the steps of: receivingthe video bit stream and decoding the video bit stream to generate adecoded video; receiving the sub-picture bit stream and decoding thesub-picture bit stream to generate at least one decoded sub-picture;capturing the size of the decoded video, and generating a first scalingfactor according to the size of the output picture and the size of thedecoded video; capturing the size and the position of the decodedsub-picture, and generating a second scaling factor according to thesize of the output picture, the size of the decoded sub-picture and asub-picture aspect ratio; and setting the size of the decoded videoaccording to the first scaling factor, setting the size and the positionof the decoded sub-picture according to the second scaling factor,combining the decoded video and the decoded sub-picture, and outputtingthe combined picture to the video output device.
 15. The video decodingmethod as specified in claim 14, wherein: the first scaling factorcomprises a first horizontal scaling factor and a first vertical scalingfactor, in which the first horizontal scaling factor is equal to thehorizontal width of the output picture divided by the horizontal widthof the decoded video, and the first vertical scaling factor is equal tothe vertical height of the output picture divided by the vertical heightof the decoded video; and the second scaling factor comprises a secondhorizontal scaling factor and a second vertical scaling factor while thesub-picture aspect ratio includes a horizontal aspect ratio and avertical aspect ratio, in which the second horizontal scaling factor isequal to the first horizontal scaling factor divided by the horizontalaspect ratio of the sub-picture aspect ratio, and the second scalingfactor is equal to the first vertical scaling factor divided by thevertical aspect ratio of the sub-picture aspect ratio.
 16. The videodecoding method as specified in claim 15, wherein the horizontal widthof the output picture is the product of the horizontal width of theactive display area of the video output device and a horizontal aspectratio of a video aspect ratio, and the vertical height of the outputpicture is the product of the vertical height of the active display ofthe video output device and a vertical aspect ratio of the video aspectratio.
 17. The video decoding method as specified in claim 14, whereinif the decoded sub-picture is for the purpose of visual effect, only thesize of the decoded video and the size of the decoded sub-picture areset when combining the decoded video and the decoded sub-picture. 18.The video decoding method as specified in claim 14, wherein the decodedsub-picture is prevented from exceeding the border of the output pictureand covering the main region of the decoded video when combining thedecoded video and the decoded sub-picture.
 19. A video decoding methodas specified in claim 14, wherein when the decoding of the sub-picturebit stream generates a plurality of decoded sub-pictures, overlay amongthe plurality of decoded sub-pictures is prevented when combining thedecoded video and the decoded sub-picture.
 20. The video decoding methodas specified in claim 14, wherein the first decoder is an MPEG decoder,and the second decoder is a run-length decoder.
 21. A video decodingapparatus capable of controlling presentation of sub-pictures, which isfor decoding a digital audio/video signal and outputting an outputpicture through a video output device, the video decoding apparatuscomprising: a first decoder programmed to receive a video bit stream andto decode the video bit stream to generate a decoded video; a seconddecoder programmed to receive a sub-picture bit stream and to decode thesub-picture bit stream to generate at least one decoded sub-picture; afirst scaler programmed to generate a first scaling factor according toa size of the decoded video and a size of the output picture; a secondscaler programmed to generate a second scaling factor according to thefirst scaling factor; and a combiner programmed to scale the decodedvideo according to the first scaling factor, to scale the at least onedecoded sub-picture according to the second scaling factor, to combinethe scaled decoded video and the at least one scaled decoded sub-pictureinto a combined picture, and to output the combined picture to the videooutput device.
 22. The video decoding apparatus as specified in claim21, wherein: the first scaling factor comprises a first horizontalscaling factor and a first vertical scaling factor, in which the firsthorizontal scaling factor is equal to the horizontal width of the outputpicture divided by the horizontal width of the decoded video, and thefirst vertical scaling factor is equal to the vertical height of theoutput picture divided by the vertical height of the decoded video. 23.The video decoding apparatus as specified in claim 21, wherein thesecond scaling factor is determined according to the first scalingfactor and a sub-picture aspect ratio.
 24. The video decoding apparatusas specified in claim 23, wherein the second scaling factor comprises asecond horizontal scaling factor and a second vertical scaling factorwhile the sub-picture aspect ratio includes a horizontal aspect ratioand a vertical aspect ratio, in which the second horizontal scalingfactor is equal to the first horizontal scaling factor divided by thehorizontal aspect ratio of the sub-picture aspect ratio, and the secondvertical scaling factor is equal to the first vertical scaling factordivided by the vertical aspect ratio of the sub-picture aspect ratio.25. The video decoding apparatus as specified in claim 22, wherein thehorizontal width of the output picture is the product of the horizontalwidth of an active display area of the video output device and ahorizontal aspect ratio of a video aspect ratio, and the vertical heightof the output picture is the product of the vertical height of theactive display area of the video output device and a vertical aspectratio of the video aspect ratio.
 26. A video decoding apparatus capableof controlling presentation of sub-pictures, which is for decoding adigital audio/video signal and outputting an output picture through avideo output device, wherein the digital audio/video signal includes avideo bit stream and a sub-picture bit stream, the video decodingapparatus comprising: a first decoder programmed to receive the videobit stream and decodes the video bit stream to generate a decoded video;a second decoder programmed to receive the sub-picture bit stream and todecode the sub-picture bit stream to generate at least one decodedsub-picture; a first scaler programmed to generate a first scalingfactor according to a size of the decoded video and a size of the outputpicture; a second scaler programmed to generate a second scaling factorfor scaling the at least one decoded sub-picture; and a combinerprogrammed to scale the decoded video according to the first scalingfactor, to scale the at least one decoded sub-picture according to thesecond scaling factor, and to combine the scaled decoded video and thescaled decoded sub-picture to generate a combined picture, and to outputthe combined picture to the video output device, wherein during thescaling of the at least one decoded sub-picture, the combiner is furtherprogrammed to determine whether the decoded sub-picture exceeds a borderof the output picture to obtain a first determining result, to determinewhether the decoded sub-picture covers a central area of the decodedvideo to obtain a second determining result, and to set a displayed sizeand a position of the decoded sub-picture according to the firstdetermining result and the second determining result, wherein if it isdetermined that the decoded sub-picture covers the central area of thedecoded video, the combiner is further programmed to re-set thedisplayed size and position of the decoded sub-picture, if it isdetermined that the decoded sub-picture does not cover the central areaof the decoded video, the combiner proceeds to set the displayed sizeand the position of the decoded sub-picture according to the firstdetermining result and the second determining result.